Multi-plane particle image velocimetry data and surface pressure measurements are used to analyze the nominally two-dimensional flow field of a high-aspect-ratio flat-plate airfoil undergoing a pure-plunge motion, as well as the three-dimensional flow field produced by a plunging flat-plate wing with aspect-ratio 2. The sources and sinks of vorticity within these flows were quantified by means of a vorticity flux analysis, the results of which verified prior conclusions that the diffusive flux of vorticity from the surface of the airfoil acts primarily as a sink of leading-edge-vortex (LEV) vorticity, with a magnitude roughly half that of the flux of vorticity introduced by the leading-edge shear layer. Inspection of the chordwise distribution of the surface diffusive flux of vorticity within the 2D case showed it to correlate very well with the evolution of the flow field. Specifically, the diffusive flux experienced a major increase during the phase interval in which the LEV was still attached to the downstream boundary layer, which ultimately triggered the roll-up of the LEV and generation of the secondary vortex. It was also noted that the accumulation of secondary vorticity near the leading edge prevented the diffusive flux from continuing to increase after the roll-up of the LEV. This result was validated through analysis of the 3D case, which demonstrated that maintaining an LEV that stays attached to the downstream boundary layer produces a larger diffusive flux of vorticity-presumably consistent with an enhancement of both lift and thrust. Despite the increased diffusive flux, this state was maintained by a strong spanwise convective flux of vorticity. This observation provides a new interpretation of the role of spanwise flow on vortex evolution, and suggests a physical mechanism that may be leveraged to control the flow.
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